Various types of heavy-duty high-current industrial equipment dissipate excess energy through resistor grids in the form of large amounts of heat. For example, resistor grids are used for controlling loads in cranes, for load testing of generators, for harmonic filtering in electric substations, for neutral grounding in industrial AC distribution, for dynamic braking on locomotives and off highway vehicles, and the like.
A resistor grid is an air or oil cooled grid of metal alloy ribbons or plates, formed into a serpentine structure. The ribbons may have engagement pins, along the length of the ribbons or plates, that are inserted into specific pin-holes in an insulation board so as to spatially restrain the ribbons within the grid. The insulation board provides a sturdy frame for the resistor grid and maintains a fixed separation between ribbons, as well as between successive grids when used in a grid stack configuration. The resistor grid provides little electrical resistance and may carry currents ranging from several hundred to thousands of amperes. Neighboring ribbons may have a potential difference of a few volts, but the voltage between parallel runs of the ribbons (or between the ribbons and electrical ground) could be hundreds to thousands of volts. Such operating parameters can cause arcing between neighboring ribbons (or to electrically grounded surfaces such as exhaust louvers) or thermal runaways if the ribbons are too close in proximity or contact one another. Therefore, the structural integrity of the insulation board is an important factor in its construction.
For example, under normal operating conditions, the resistor grids are typically subject to air temperatures between 200 and 400 degrees centigrade. These high temperatures may cause thermal degradation and/or distortion of portions of the insulation board. If portions of the insulation board distorts or degrades, then the pins may be allowed to move from their keyed positions in the insulation board. This may further lead to relative motion of the ribbons, ribbon-to-ribbon contact, electrical arcing, thermal runaway, and subsequent deterioration and ultimate failure of the resistor grid. Accordingly, it is now recognized that materials of construction for the resistor grid insulation boards should exhibit a balance of thermal, mechanical, and abrasion resistance to provide desired reliability.